A Planar Lightwave Circuit (PLC) is an optical system comprising one or more integrated-optics waveguides that are integrated on the surface of a substrate, where the waveguides are typically combined to provide complex optical functionality. These “surface waveguides” typically include a core of a first material that is surrounded by a second material having a refractive index that is lower than that of the first material. The change in refractive index at the interface between the materials enables reflection of light propagating through the core, thereby guiding the light along the length of the surface waveguide.
PLC-based devices and systems have made significant impact in many applications, such as optical communications systems, sensor platforms, solid-state projection systems, and the like. Surface-waveguide technology satisfies a need in these systems for small-sized, reliable optical circuit components that can provide functional control over a plurality of optical signals propagating through a system. Examples include simple devices (e.g., 1×2 and 2×2 optical switches, Mach-Zehnder interferometer-based sensors, etc.), as well as more complex, matrix-based systems having multiple surface waveguide elements and many input and output ports (e.g., wavelength add-drop multiplexers, cross-connects, wavelength combiners, etc.).
Common to most of these systems is a need for a switching element. Historically, the most common switching elements suitable for use in a PLC are based on a device known as a thermo-optic (TO) phase controller. A TO phase controller takes advantage of the fact that the refractive index (i.e., the speed of light in a material) of glass is temperature-dependent (referred to as the thermo-optic effect) by including a thin-film heater that is disposed on the top of the upper cladding of a surface waveguide. Electric current passed through the heater generates heat that propagates into the cladding and core materials, changing their temperature and, thus, their refractive indices. TO phase controllers have demonstrated induced phase changes as large as 2π.
To form an optical switching element, a TO phase controller is typically included in a surface waveguide element, such as a Mach-Zehnder interferometer (MZI). In an MZI switch arrangement, an input optical signal is split into two equal parts that propagate down a pair of substantially identical paths (i.e., arms) to a junction where they are then recombined into an output signal. One of the arms incorporates a TO phase controller that controls the phase of the light in that arm. By imparting a phase difference of π between the light-signal parts in the arms, the two signals destructively interfere when recombined, thereby canceling each other out to result in a zero-power output signal. When the phase difference between the light-signal parts is 0 (or n*2π, where n is an integer), the two signals recombine constructively resulting in a full-power output signal.
Unfortunately, prior-art PLC-based switching elements have disadvantages that have, thus far, limited their adoption in many applications. First, TO phase controllers consume a great deal of power. Further, in addition to heating the core and cladding materials directly below the heater element, heat from the thin-film heater also diffuses laterally in the glass, which can lead to thermal crosstalk between adjacent surface waveguides. Still further, glass has a low thermal conductivity coefficient, which results in heating and cooling times that are long (typically, on the order of milliseconds). Thermal crosstalk also limits the density with which heating elements can be formed, limiting the number of TO phase controllers that can be included on a single chip. As a result, TO phase controllers are poorly suited for many applications.
More recently, the photo-elastic effect has been exploited as an alternative to thermo-optic tuning of the refractive index of the materials of a surface waveguide. Phase shifting of a light signal in surface waveguides based on the photo-elastic effect was disclosed, for example, by S. Donati, et al., in “Piezoelectric Actuation of Silica-on-Silicon Waveguide Devices,” published in IEEE Photonics Technologies Letters, Vol. 10, pp. 1428-1430 (1998), and by Tsia, et al., in “Electrical Tuning of Birefringence in Silicon Waveguides,” App. Phys. Lett., Vol. 92, 061109 (2008), and in U.S. Pat. Nos. 9,221,074 and 9,764,352, each of which is incorporated herein by reference. While phase shifting on the order of a microsecond with low power dissipation was demonstrated, the efficiency with which a phase change could be induced in the constituent layers (particularly the core layer) of the surface waveguides was poor. As a result, very high voltages and large interaction lengths were required, which limits the utility of prior-art photo-elastic-based phase tuning in practical PLC systems.
The need for an efficient integrated-optics phase tuning technology that enables fast, low-power-consumption operation remains, as yet, unmet in the prior art.